I run DoctorDiff's 15/16" on my Duster and my Challenger. Duster has been set up with 11.75" 73+ disks and 11x2.5" rear drums with that master cylinders, and currently runs 13" Doctor Diff cobra style front disks with DoctorDiff 11.7" rear disks. The Challenger currently has the 11.75" front disks and 11x2.5" rear drums.
The 15/16" master cylinder is the way to go. Pedal effort isn't too heavy, the slightly longer stroke allows for better braking control/modulation without being too long of a stroke to put you in danger of putting the pedal on the floor.
Which Wilwoods?
Honestly I wouldn't do Wilwood anything for the price. I have the DoctorDiff 13" Viper kit I will be swapping onto my Duster, the 13" cobra style kit I run now was only a minor improvement compared to the 11.75's I had on there before. The other option would be to contact the guy at Invincible Extremes Muscle Car Garage that does the brackets for the GT500 calipers, you can get anything from 13" to 15" rotors that I've seen so far.